1. Field of the Invention
The present invention relates to an image processing system, and particularly to a method for dynamically scanning an image.
2. Description of the Prior Art
An image signal processing system electrically generates an image signal through focusing a scanned light beam and using a photodetecting device such as charge-coupled devices (CCDs). The generated image signal is then further processed, stored and displayed for some applications. Image scanners, camera recorders and facsimile machines are some examples among the widespread applications of the image signal processing systems in modem offices and home.
A functional block diagram of a conventional image system is shown in FIG. 1. An image of a document 10, which is illuminated by a lamp 12 and reflected by a mirror 14, is focused by at least a set of lens 16. The focused image is then detected by a CCD 18, followed by being processed via a pre-processing circuit 20 such as a direct current (DC) gain amplifier, which is usually implemented by an operational amplifier. Thereafter, the pre-processed signal is transformed from its analog form to an equivalent digital form by an analog-to-digital converter (ADC) 22. A black reference input R.sub.B and a white reference input R.sub.w are input to the ADC 22 to adjust the range of the system response. The digitized signal is further processed by a post-processing circuit 24, such as a Gamma correction, and is then stored in a memory buffer 26 for further processing or display.
Density range is one of the important performance parameters of an image processing system such as a scanner. This density range is closely related to a signal-to-noise (S/N) ratio in the system. For example, a scanned image having a density range of two (2) can be distinguished only provided that a S/N ratio is greater than 100 (i.e., 10.sup.2). Generally, an image processing system is said to have a density range D if the S/N ratio in the system is greater than 10.sup.D. The S/N ratio mentioned above is primarily determined by a S/N ratio of a charge-coupled device (CCD) and/or a S/N ratio of the other portion ofthe system. For a digital image processing system, the S/N ratio is related to the number of system bits. For example, for a system having 256 levels of gray, its S/N ratio becomes 256, and its density range D thus becomes 2.4 (i.e., log 256).
For the conventional image processing system as described in connection with FIG. 1, unfortunately the image signal processed by the system is usually distorted by the non-linearity of some devices, such as the operational amplifier 20 and the CCD 18. A characteristic response of a conventional CCD is shown in FIG. 2A, where X axis represents the reflectivity or light intensity received by the CCD, and Y axis represents the output voltage of the CCD. Dashed line 30 indicates an ideal linear response of the CCD, and curve 32 shows the response of a commercial CCD device. Section 32a on the curve 32 shows the dark portion of the image, and section 32c shows the bright portion of the image. As can be seen from the diagram, only section 32b on the curve 32 possesses a linear response.
FIG. 2B further shows a response of system density where the input density and the output density are represented by X axis and Y axis respectively. Section 34a on the response curve 34 shows the dark portion of the image and section 34c shows the bright portion of the image. Further, section 34b on the curve 34 has a linear response.
For the conventional image processing system as described above and shown in FIG. 1, most parameters for performing scan are fixed, or at least not conveniently adjustable. These parameters include exposure time, intensity of the exposure, reflectivity to the CCD 18, value of the dc gain amplifier 20, black reference and white reference inputs to the ADC 22, value of Gamma in the post-processing circuit 24, or highlight/shadow in the post-processing circuit 24.
FIG. 3 is a histogram (or spectrum) illustrating two worst-case examples, where the value on the X axis represents the level of gray, and the value on the Y axis represents the total count of occurrence for a specific gray level. Curve 36 indicates a dark image and curve 38 indicates a bright image. However, the conventional image processing system scans these two images with the same scan parameter, resulting in a poor scan on both images.